Circuit for purifying high frequency radiation



FESOA/A IVCE COIL -A. H. TAYLOR 1,875,952

CIRCUIT FOR PURIFYING HIGH FREQUENCY RADIATION Sept. 6, 1932.

Filed Sept. 20, 1926 'INVENTOR. Wad/t 3K, 921%;

ATTORNEY Patented Sept. 6, 1932 warren stares PATENTQFFICE 7 i ALBERT n.TAYLOR, or WASHINGTON, msTRIoT on CQLUMRTA, AssIeno BY MESNEASSIGNMENTS, 'IO WIRED- RADIO, INC;

DELAWARE on NEW YORK, N. Y., A CORPORATIQN or CIRCUIT FOR PUBIFYING HIGHFREQUENCY RADIATION Application filed'Scptember 20, 1926. Serial No.136,702.

'My invention relates broadly to high frequency signal transmittingsystems and more particularly to a circuit arrangement for mittingsystem having special circuit arrangements for eliminating the evenharmonics of the crystal frequency and greatly reducing the oddharmonics forsecuring relatively large radiation at the crystalfrequency by reducing losses on other frequencies.

Still another object of my invention is to provide a coupling system fora frequency multiplier circuit with an antenna system for delivering tothe antenna the desired signaling frequency free of interference fromundesired frequencies.

My invention will be more clearly understood from the followingspecification by reference to the accompanying drawing, in which: V

Figure 1 illustrates theprinciple of my invention embodied in a piezoelectric crystal frequency controlled transmitter; and Figs. 2 and 3 arediagrammatic representations of the current and voltage characteristicsin high frequency signal radiating circuits, Fig. 3 showing theadvantages obtained in the circuit of my invention over. circuitsheretofore employed and having characteristics as shown in Fig. 2. V

The simplest way to operate an antenna on high frequencies is to work itclose to the fundamental. Thusthe current will be maximum at or near thebase of the antenna and the voltage will be minimum at or near the sameplace. An ammeter in series with the antenna will then readily indicateresonance and the efiective resistance of the antenna will be relativelylow permitting a fairly loose couplingto be used with the transmitter.

It often happens that such an antenna, unless erected in a clear openspace not surrounded by absorbing structures, will not be an efficientantenna. The direction'of maximum energyis', moreover, almost parallelto the ground whereas long distance work re-. quires maximum use to bemade of high angle components that come down after reflection from theKennelly-Heaviside layer and produce the received signals; Therefore itis usually advisable to use alonger antenna and inasmuch as it is notalways convenient to change the length of this antenna when changingwaves, certain waves will be found for which the voltage and currentdistribution in the antenna is similar'to that shown in Fig. 2. 1

.. The voltage may be high at the top and high at the bottom, whereasthe current is maximum near the center of the antenna and zero at thebottom. The same situation arises in an antenna which is an integralmultiple of the length of the one illustrated in this figure. Little orno current will be shown at the base of such an antenna and it isdiflicult to know when maximum energy is being put into it except byindirect observation. Moreover, the effective resistance of. the antennameasured at its base is extremely high, which is the reason why fromanother point of view the antenna requires a very tight coupling to thetransmitter;

In high frequency work manysets working on a frequency higher than 4,000kilocycles, use a frequency multiplier in association with theoscillation system. If, for instance, the antenna is radiating 16,000kilocycles, the chances are, at least with many types of design, thatthere will be another frequency somewhere in the set of 8,000 and stillanother at 4,000. 'That is, one or more stages of the power amplifiermay be simultaneously acting as frequency multipliers. The factor ofmultiplication is sometimes 2, sometimes 3, and sometimes some othernumber although high efliciency'is not ordinarily obtained whenthefrequency multiplication factor in any particularamplifier higherthan 3. The result of this situation is that such a transmitter is notonlyprone to put harmonics into the antenna, but if theantenna is verytightly coupled to the transmitter, it may also put in some of theselower frequencies which might be called sub-harmonies. his is an unusualsituation and does'not occur, except where the transmittershavefrequency multipliers.

' When a frequency is desired corresponding to a half-wave antenna suchas is illustrated in Fig. 2, or if the antenna is an integral multipleof the length of such a one, it is relatively difiicult to change thefree wave length antenna, an inductance shunted by a variable capacityso that excitation of the antenna will result in a strong circulatingcurrent in this capacity-inductanceparallel combination, but such acurrent is wasteful of energy. This method has been used, but it is noteificient, convenient, or easy to adjust. r

' I have discovered that it is possible to load the base of the antenna,not with a .pure inductance nor with a pure inductance shunted with anadjustable capacity, but with an inductance containing intentionally aconsiderable amount of distributed capacity. Such an inductance operatesas aresonance'coil, since standing waves'may be readily produced in itand indeedwith very short overall linear dimensions.

Reference character 1 designates an electron tube having cathode 2, gridelectrode 3 and plate electrode 4 arranged in oscillatory Y circuits forthe generation of high frequency oscillations. The cathode 2 is heatedfrom a suitable source 5 under control of rheostat 6. The input circuitof the oscillator is represented at 7 under control of the piezoelectric crystal circuit comprising the piezo electric crystal element 8disposed between conductive plate members 8a and'8b in shunt withrespect to the choke coil 9. The output circuit of the electron tube 1is connected through condenser 11 with inductance 10, which is tunedpbymeans ofv condenser 12 with'anammeter 13 "connected in circuit therewithfor observing the current values in the circuit. A power amplifier orfrequency multiplier system is connected to the tunedcircuit includinginductance 10 and comprises the electron tube -14 having cathode 15,grid electrode 16 and plate electrode 17.' The cathode 15 is heatedfrom'battery 18 under control o f'r heostat 19.

A connection to the input circuit of electron tube 14 is'made throughcondenser 20. The

output circuit of the power amplifier connects through choke coil 24:.The plate'supply for the power amplifier or frequency multiplier tube 1L is obtained from the generator 22 through key ng circult 25 an dcliche coil 23.

the power amplifier tube 14 between grid electrode 16 and cathode 15 forinsuring the proper potential for the grid of the electron tube let. Theresonance coil is designated at30 having a tap 31 thereon which isconnected to the'end of the coil through'conductor 32 and to theinductance 26 through a lead which in? mudes ainmeter 33. V The antennamay be connecteddirect to the top of the coil'or may be coupled looselyto it through a small variable condenser 34 arranged to be shunted bymeans of switch '35 as shown. The rest of this coil maybe adjusted by amovable tap preferably with the arrangement as shown in the drawing, toshort circuit thev unused part of the coil, thus hindering standingwaves from being developed in the lower portion of the coil. The upperportion of the coil then isthe portion which is made sharply resonantto' siderable amount of current and can be read- 'ily used to mdicateresonance.

The potential distribution when the condenser is used is shown in Fig.3, the potential being low at the bottom of the coil and current beinghigh. This makes the system very convenient andeasy to handle, permitsloose coupling to the transmitter, and due to the great sharpness oftuning at the resonance coil 30 not only takes out the even harmonics,but greatly reduces the odd ones also, and prevents altogether thefrequencies lower than the fundamental reaching the antemia. I havefound the circuitarrangement described herein particularly suitable forhigh frequency transmission. The interposition of the resonance coilprevents losses due to spurious currents both ofhigher and lowerfrequencies thanthe one desired. 'By'observing a transmittercircuit'with a wave meter with theresonance co1l-1n the circuit 'A chokecoil 2i bridges the input circuit of and removed from the circuit, Ihave found that the transmitting circuit without the resonance coiltherein shows the existence of considerable antenna current offrequencies other than the one desired, which current is normally partof the loss in the circuit, but with the resonance coil inserted in thesystem the radiation is purified by the elimination of these undesiredfrequencies, with a resulting increase in the radiation of the desiredfrequency.

While I have described my invention in certain particular embodiments, Idesire that it be understood that modifications may be made and that nolimitations upon my invention are intended other than are imposed by thescope of the appended claims.

lVhat I claim as new and desire to secure by Letters Patent of theUnited States is as follows:

1. In a high frequency transmission system, a source of waves capable ofsupplying waves of different frequencies including waves of a desiredfrequency, a radiating conductor having a length equal to one half thewave length of the desired waves, and means for coupling said source tosaid conductor for the radiation of waves of the desired frequency tothe substantial exclusion of waves of other frequencies comprising aconnecting link having an electrical length of an odd multiple of thequarter wave length of the desired waves.

2. A system as described in claim 1 further characterized in that acondenser is inserted between said connecting link and said radiatingconductor.

3. In a high frequency transmission system, a piezo crystal controlledmaster oscillator, means for developing harmonics of the wave producedby said oscillator, said means ineluding a circuit tuned to thefrequency of one of said harmonics, an antenna system comprising aradiating conductor one half wave length long connected to ground orcounterpoise through a resonance coil having an effective electricallength of an odd multiple of one-fourth wave length of said oneharmonic, and means for transferring energy from said tuned circuit tosaid radiating conductor through said resonance coil.

4. In a high frequency transmission system which comprises a source ofwaves of desired and undesired freqencies, a radiating conduc-- tor anda transmitting link having an electrical length equivalent to an oddmultiple of a quarter wave-length of the desired frequency, the methodof suppressing the radiation of the undesired waves, which comprisesapplying energy to said radiating conductor at an intermediate pointthereof, loading the base thereof with an inductance and a considerableamount of distributed capacity and establishing in said radiatingconductor standing waves the voltage nodal point of which is at the basethereof.

5. In a high frequency transmission system which comprises a source ofwaves of desired and undesired frequencies, a radiating con' ductor andan inductance coil connected thereto having a relatively largedistributed capacity between the turns thereof, the method ofsubstantially suppressing the undesired frequencies and radiating onlythe desired frequency, which consists in establishing in said inductancecoil a condition of resonance to the undesired frequencies, and infixing a voltage nodal point at the base of said radiating conductor.

6. The method as set forth in claim 5, which includes the further stepof characterizing the electrical length of said inductance coil so thatit becomes resonant to an odd multiple of a quarter wave-length of thewave of desired frequency.

ALBERT H. TAYLOR.

